1. Field of the Invention
The present invention relates to a semiconductor device that has a semiconductor element (specifically, a thin film transistor), and also relates to display devices such as an EL display device (a light emitting device) provided with a light emitting element and a liquid crystal display device and a manufacturing method there of, especially, to a large-sized display device and a manufacturing method thereof.
2. Description of the Related Art
These days, it has been proceeded to make a screen have a large size and a high definition in active matrix semiconductor devices such as an EL display device and a liquid crystal display device, and the number of wirings such as signal lines and scan lines tends to increase as well as a length of the wiring. Therefore, it is required to prevent voltage drop due to wiring resistance, trouble in writing of a signal into a pixel, and trouble in gray scales.
In the EL display device, a driving method for displaying an image with multiple gray scales has a voltage input system of inputting voltage from a signal line and a current input system of inputting current from a signal line, and further, the voltage input system has a current writing system of writing data in current format (driving current) into a light emitting element for controlling luminance and a voltage writing system of writing data in voltage format into a light emitting element for controlling luminance while the current input system has the current writing system. Such driving method especially has trouble caused due to wiring resistance. The wiring resistance causes delay in transmission of a signal to a terminal of a signal line, and the voltage drop of a power source line (specifically, a current supply line) makes it difficult to supply predetermined current or voltage. Then, fluctuation in displaying is caused as the result since luminance of light emitted from the light emitting element is proportional to a value of the current or voltage supplied through the signal line. Further, due to the voltage drop, the EL display device and the liquid crystal display device have trouble of waveform distortion of a pulse signal input through the wiring.
In the case of using copper as a material with low resistance for forming a wiring, there is a means of forming a plugged wiring (a structure formed with damascene). In the damascene, however, an insulating film is provided at the position of a wiring to be formed, a groove for forming the wiring is formed in the insulating film, and then a Cu wiring is formed in the groove (for example, Japanese Patent Laid-Open 2000-58650).
Therefore, in conventional display devices, beaten-copper with low resistance is used for a PWB-side wiring of a printed-wiring board (PWB) in electrically connecting the PWB-side wiring to an element-side wiring with an isotropic conductive film instead of a wiring to suppress voltage drop of the element-side wiring and signal delay (for example, Japanese Patent Laid-Open 2001-236025).